Such drilling machines have been known for a long time. They include a mount arranged in axial extension of the tap-hole. A drill drive is slidably arranged on the mount. This drill drive causes a tap-hole drill to rotate, generates forward feed and withdraws the tap hole drill from the tap-hole. For large blast-furnaces such tap-hole drills can be as much as four meters long. They consist essentially of a smooth drill-rod carrying a drill head at its front end. The rear end of the drill rod is clamped in a chuck of the drill-drive.
At its front end, the mount has a protective shield of refractory material. The main function of the protective shield is to protect the mount and the drill drive against the molten metal streaming out of the drilled tap-hole. Another function of the protective shield is to guide the tap-hole drill during the drilling operation.
When the tap-hole is opened, the protective shield regularly comes into contact with the molten metal. The protective shield is thereby heavily stressed, particularly in the region of its guide hole for the tap-hole drill, so that, in a comparatively short time, the protective shield can no longer guide the drill rod properly. In order to re-establish proper guidance of the drill rod, the protective shield has to be replaced, even though it is still capable of performing its protective function.
EP-A-0519397 discloses a tap-hole drilling machine which has a special guiding means for the tap-hole drill at the front end of its mount. This guiding means is formed by two guide sections made from steel or cast iron, each being pivotably attached to the mount by means of a lever arm. A hydraulic or pneumatic cylinder attached to the mount is connected to the two lever arms by means of a linkage system, so that it can pivot the two guide sections from a working position, in which they cooperate to form a drill guide for a tap-hole drill, sideways to a rest position. In this rest position, the two guide sections are located to the side of the mount, behind a protective shield attached to the front face of the mount. In the device described in EP-A-0519397, the front protective shield performs only the function of protecting the two guide sections in the rest position. Because of its wide aperture in the region of the drilling axis, the front protective shield is actually not capable of protecting the mount and the drill drive against the molten metal issuing from the drilled tap-hole. It should also be pointed out that, despite the protective shield, the guiding device at the end of the mount represents a very exposed unit, so that damage cannot be completely ruled out. It must therefore be ensured that even serious damage to the guiding device must not cause an excessively long down-time of the tap-hole drilling machine. This assurance does not exist, however, with the tap-hole drilling machine described above.
The present invention intends to eliminate or reduce the above-mentioned disadvantages of the prior art tap-hole drilling machines.